A comparison of DA white dwarf temperatures and gravities from Lyman and Balmer line studies

We present measurements of the effective temperatures and surface gravities for a sample of hot DA white dwarfs, using the Lyman line data available f rom the HUT ORFEUS and FUSE FUV space missions. Comparing the results with those from the standard Balmer line technique, we find that there is a gene ral good overall agreement between the two methods. However, significant di fferences are found for a number of stars, but not always of a consistent n ature in that sometimes the Balmer temperature exceeds that derived from th e Lyman lines and in other instances it is lower. We conclude that, with th e latest model atmosphere calculations, these discrepancies probably do not arise from an inadequate theoretical treatment of the Lyman lines but rath er from systematic effects in the observation and data reduction processes, which dominate the statistical errors in these spectra. If these systemati c data reduction effects can be adequately controlled, the Lyman line tempe rature and gravity measurements are consistent with those obtained from the Balmer lines when allowance is made for reasonable observational uncertain ties.